...Model guidance was generally good and forecasters took advantage of
the good forecast data and their meteorological reasoning to issue Winter Storm Watches which
verified with an average lead time of nearly 40 hours. Winter Storm Warning lead times verified around 18 hours with
no missed events...
...Isolated bands of snow that moved across the Northern Piedmont of North Carolina
during the morning of March 2, 2009 appeared to be a result of thermal instability in the boundary
layer that produced horizontal convective rolls...
...Forecasters used NCDOT and other highway traffic cameras to supplement the limited traditional
observations during the storm. Collecting information on snow accumulation and snow impact during the
overnight hours is often very difficult...
...Forecasters utilized AMDAR aircraft soundings to provide thermal profiles at GSO and RDU in between
RAOB times. This data set is an important resource during critical winter weather forecasts.
...Several CoCoRaHS intense snow reports were received by the NWS Raleigh. These reports are
especially important since it is very difficult to get accurate snow accumulation reports late at
night...
...Warm weather preceeding the evwnt along with warm ground temperatures
resulted in some melting of the snow as it accumulated across the Eastern Piedmont and Sandhills...
...The CIPS Winter Storm Analog Guidance data was archived during the event and a brief
summary of the methodolgy and the data for this event are available.
...A notable mountain shadow effect can be seen in the snow accumulation
analysis and in the high resolution MODIS visible satellite picture...

Snow accumulations were generally stratified from northwest to southeast primarily because of the
temperature distribution in the lower portions of the atmosphere and the placement of the
greatest forcing for ascent. Outside of the mountains where 12 inches or more of snow was reported, the heaviest snowfall amounts
of 6-10 inches fell in a discontinuous band running from west of Charlotte to west of Greensboro. An average of 2 to 4 inches of snow fell across the eastern portions of the
Piedmont and an inch or less fell in the Coastal Plain.

Across central North Carolina, the greatest snowfall reports were of 7 inches in Person
County. Snow accumulations generally decreased from west to east with 4-6 inches of snow
in the Triad, 2-4 inches along the U.S. Highway 1 corridor and a Trace–2 inches
falling along the I-95 corridor. Officially, the RDU airport measured 3.2 inches of snow and
Greensboro had 5.7 inches.

A Java Loop
of surface analysis imagery from 00 UTC Saturday, February 28 through 00 UTC on
Tuesday, March 3, 2009 shows the evolution of event.

Satellite Imagery

Water vapor imagery was used to monitor the track and intensity of the
upper low as it moved across the mid Mississippi Valley into the Deep South and
then across the Carolinas. The Water vapor imagery was also used to monitor the
development and progression of a significant dry slot that moved across the Carolinas
from around 18 UTC on March 1 through around 03 UTC on March 2. The dry slot
produced a significant break in the
precipitation across the region on Sunday evening.

A Java Loop
water vapor imagery from 1215 UTC on Saturday, February 28 through 1815 UTC on Monday,
March 2, 2009 is available.

Southeast Regional Radar Imagery

The regional radar imagery shows the multiple rounds of precipitation
that moved across the region during the event. Each area of
precipitation was generated by different forcing mechanisms.
For example, the first area of precipitation was produced by
isentropic lift or warm advection
over the southeastward advancing cold air mass that
arrived across North Carolina during the early morning hours on Saturday, February 28
and persisted into the mid afternoon hours. Another area of rain advanced into
central North Carolina just before daybreak on Sunday, March 1. This area
of precipitation was driven by significant upward motion ahead of the approaching upper
level low including low level warm advection, and
impressive upper level divergence. This area of
precipitation diminished as a pronounced dry slot
(water vapor imagery from 2115 UTC on March 1
| 700 mb analysis from 21 UTC on March 1)
moved across the Carolinas between 18 UTC on March 1 and 03 UTC on March 2.
Finally, a third area of precipitation moved across North Carolina during the evening hours
on March 1 and overnight into the morning hours of March 2. This area of precipitation was
associated with the deformation zone located to the north and northwest of the
850 mb and 700 mb low track.

A Java Loop of
Southeast regional radar imagery from 2358 UTC on Friday, February 27
through 1458 UTC on Monday, March 2, 2009 is available.

Partial Thickness Values

The thickness of a layer of air is proportional to the layer's mean
temperature. The warmer the layer, the greater it's thickness. The layer
of air bounded by a pressure of 1000 - 850 MB is used by forecasters to
monitor the average temperature in the lower level of the atmosphere
(roughly from the surface to 5,000 ft). The
850 - 700 MB layer is closely followed by forecasters to monitor the
average temperature in the layer of air (roughly from 5,000 to 10,000 ft)
where elevated above freezing temperatures may
exist. NWS forecasters at Raleigh have for decades now, correlated the
thickness of these layers as observed in RAOB's to
the observed wintry precipitation types. The technique has proven to be
helpful in anticipating the frequent precipitation type changes often
associated with winter storm events in central North Carolina.

A Java Loop
of partial thickness and weather from 12 UTC February 28, 2009 through 12 UTC March 2, 2009 is available.

TREND’s Predominant P-type Nomogram

The nomogram below shows the distribution of precipitation (p-type) TREND's as a function of partial
thickness values. Close examination of precipitation events over the past 30 years
accounts for the boundaries on the nomogram separating the various p-type areas.
Mid level thickness values increase from left to right along the x axis. Low level thickness
values increase from bottom to top along the y-axis.

The first image below displays the observed thickness values from the 6 hourly RAOB's
at KGSO from 01/00 UTC through 02/00 UTC on March 2nd. Initially the atmosphere
cooled in the lower levels as shown in the nearly vertical drop in the TREND's line produced by a
significant drop in the 1000-850 mb thickness between 12 UTC on 2/28 and 00 UTC on 3/1.
The lower levels of the atmosphere do not cool much between 00 UTC and 12 UTC on 3/1 with some
cooling in the mid levels indicated by a drop in 850-700 mb thicknesses. More significant
cooling occurs between 12 UTC and 18 UTC with the air mass changing little
between 18 UTC and 00 UTC on 3/2. A significant drop in the 1000-850 mb and 850-700 mb thicknesses occurs
after 00 UTC on Tuesday as the upper low approaches. The second image contains a
list of observations from KGSO from 12 UTC on 2/28 to 12 UTC on 3/2. Note the changeover
from rain to snow that occurs around 00 UTC to 01 UTC on 3/2.

A previous study, Snowbands
during the Cold-Air Outbreak of 23 January 2003 (Schultz, D.M., D.S. Arndt, D.J. Stensrud, and J.W. Hanna, 2004)
investigated similar events and hypothesized that
the bands were produced through two processes: 1) thermal instability
in the planetary boundary layer that produced horizontal convective rolls (HCR's) over widespread areas, and 2)
lake-effect processes downstream of small lakes which produced localized bands. The horizontal convective rolls
are a strong candidate for the production of the bands since the bands were associated with
a strong cold-air outbreak, at times they were regularly spaced over
a large area, and occurred within the planetary boundary layer. The study noted that the updrafts associated
with HCR's in the planetary boundary layer can saturate and produce parallel cloud bands or cloud streets
if the updrafts are sufficiently deep and moist. HCR development can be result of either thermal or
dynamic instabilities. The March 2, 2009 event was associated with cold advection over
a relatively warm land surface suggesting that the thermal instability mechanism likely produced the HCR.
The same study noted that for this mechanism, the buoyancy provides the
energy for the circulations, and the vertical wind shear organizes the circulations into bands provided the
instability does not become too great. Finally, HCR's due to the thermal instability mechanism are
frequently observed in slightly unstable environments with some sensible heat flux from the surface.

In the study by Schultz, Arndt, Stensrud, and Hanna, they
state that HCR's in the planetary boundary layer form when the
environmental lapse rate is near neutral or slightly unstable,
the mean wind speed in the roll layer exceeds
some relatively small value (2–5 m/s), the vertical
wind shear is nonzero, and a modest value of surface
sensible heat flux exists (Atkinson and Zhang, 1996).

The four panel image of RUC analysis soundings shown to the right (click on
the image to enlarge) for KRDU on March 2 at 09 UTC, 12 UTC, 15 UTC, and 18 UTC
shows that the boundary layer became slightly unstable. In fact, the
RUC soundings at 12 UTC and
at 15 UTC indicates that a layer
at or just above the surface which was around 4,000 to feet deep had become unstable with
the top of the boundary layer at around 850 mb or 5,000 feet.

The bands of light snow during the morning and into the midday hours on Monday generally produced
only light snow accumulations with most locations in the Northern Piedmont reporting an
additional light coating of snow on Monday morning. The snow was rather persistent
through much of the morning across portions of the area (observations from Raleigh-Durham
and Henderson) with light snow falling for several hours
in North Raleigh with only a light dusting of new snow observed. This period of snow was generally
unanticipated and was likely a surprise to many residents in the area.

Another interesting aspect of the snow bands is that some of the bands appear to have at least a limited
connection with the relatively large lakes that straddle the North Carolina - Virginia border
including Kerr Lake and Lake Gaston. The reflectivity loops from the
KRAX WSR-88D radar located 10 miles southeast of Raleigh
and the TRDU Terminal Doppler Weather Radar (TDWR)
located just north of the Raleigh-Durham International Airport can be stopped and advanced to allow
improving viewing of the phenomenon. While many of the bands do no appear to originate or even be
located downwind of the lakes, there are a couple of bands that may be enhanced by the additional
flux of moisture heat from the relatively mild lake surface.

Oftentimes the precipitation on the back edge of a departing winter cyclone ch0nages from
light snow to a light drizzle or freezing drizzle as the deep moisture exits and the
only lingering moisture is shallow and below the favored growth zone for dendritic ice
crystals between -12 &deg C and -18 &deg C. The RUC analysis soundings from March 2 at KRDU
indicate that for much of the morning after daybreak the low level moisture was generally confined below
(warmer) the -10 &deg C to -12 &deg C layer (RUC soundings for KRDU on March 2 at
09 UTC, 12 UTC,
15 UTC, and 18 UTC.
The moisture appeared to be sufficient at 09 UTC with
considerable drying noted at 12 UTC. The low level moisture
extended up to -14 &deg C at 15 UTC with only some slight drying
in the sub -10 &deg C layer at 18 UTC.

The bands of snow that moved across the Northern Piedmont of North Carolina
during the morning of March 2, 2009 were an unusual event. The
bands may be a result of thermal instability in the boundary layer that produced horizontal convective
rolls (HCR's). The instability is largely the result of large scale cold advection over a relatively warm
ground that produces a modest sensible heat flux. The HCR's may have been enhanced by additional fluxes of sensible heat and moisture from the
relatively large lakes near the North Carolina - Virginia border including Kerr Lake and Lake Gaston. It
is somewhat uncommon in winter across central North Carolina to get the conditions necessary for snow producing
HCR's including a boundary layer that is very moist, increasingly unstable, and sub freezing.

The goal of the analog forecast approach is not to make a forecast; but to provide
medium-range guidance for events by using a historical dataset. In addition, a forecaster can
quickly gain historical experience and become familiar
with the meteorological patterns associated with certain events. The analog forecast
approach can be applied to any meteorological event as long
as a control run can be created.

During the first pass through the NARR dataset, the statistics are computed on a large domain (REGN)
and then on a smaller domain (MESO). If certain thresholds are not exceeded, the date/time is not considered
a potential analog.

To reduce the approximately 20,000 potential analogs, threshold values were determined
based on the control run for the following fields:

Once the approximately 20,000 potential analogs are reduced, “duplicate” times are removed.
“Duplicate” times occur due to the variability in system speed (e.g., a slow historical
system may exhibit similar patterns to the forecast over a longer period of time).

The best analog is found over a 24-h period by using the following formula:

SUM(COR) - SUM(MAE/3)

After the potential analogs are reduced, the program is
rerun to find statistics on the following variables:

After new statistics are determined, a results score is computed using the following formula:

850HGHTCOR*3 + PMSLCOR*2 + SUM(COR) - SUM(MAE/3)

Finally, in order to catch possible system propagation, statistics are computed for ± 12 h from the
time of the best analog using the matching forecast (the 48 hour forecast is used in the example below):

The final analog rank is determined by using the results scores in the following formula:

AVERAGE(m12ANALOG, ANALOG, p12ANALOG)

Example Analog for this Event

The CIPS Heavy Snow Analog Guidance for 00 UTC on 3/2 across the
East Coast 2 domain
was examined for this event and a brief review of the process and
data is shown below. The analog was based off the 48 hour forecast
from the 40km GFS (212 grid) valid at 00 UTC on 3/2. The
four panel GFS 48 forecast indicated
an impressive 500 mb cutoff low centered just west of Charleston, SC (lower left panel) and
a modest 1004 mb surface low off the North Carolina coast with a band of snow falling
across the western Carolinas and Virginia (upper left panel).

Total number of potential analogs: 20044
Number of potential analogs that did not exceed set thresholds: 19954
Number of potential analogs that exceeded set thresholds: 90
Number of distinct analog events: 42

The 19890223/1800 analog had the greatest final analog rank based on the averaging of that forecast along with
the statistics from 12 hours prior to and after the 0223/18 forecast. The CIPS Heavy Snow Analog page provides
analysis maps of the COOP snowfall from the various analogs along with some
probabilities based on the analogs.

The COOP snowfall analysis maps from the CIPS Heavy Snow Analog page for 02/23/1989, 02/24/1989, and 02/25/1989 are
shown below. In addition, the mean
and median of the top
15 analogs are also provided on the CIPS Heavy Snow Analog page.

Some of the more insightful products are the
probabilistic forecasts of COOP snow greater then various thresholds. The
probabilistic forecast of COOP snow greater then 2 or 4 inches based on the
top 15 analogs is shown below. Additional probabilities for COOP snow amounts greater then
6 or 8 inches are also available along with probabilities of snow to liquid ratios
from COOP observations greater then 10:1, 12:1, 14:1, and 16:1.

AMDAR Aircraft Soundings

AMDAR is an acronym for Aircraft Meteorological DAat and Reporting (AMDAR) which is an international
effort within the World Meteorological Organization to coordinate the collection of
environmental observations from commercial aircraft. In the United States, we often refer to the
Meteorological Data Collection and Reporting System (MDCRS) which is a private/public partnership
facilitating the collection of atmospheric measurements from commercial aircraft to improve aviation safety.

AMDAR is very useful for short term forecasting situations where conditions are changing rapidly and in particular for aviation forecasting. Regarding winter weather events,
AMDAR data can provide forecasters with the height of the freezing level, the presence of elevated warm layers,
indications of thermal advection and dry layers. All of these are necessary for
accurate precipitation type forecasts. The availability of this upper air data at times
and locations where RAOB data may be lacking is invaluable.

The image below contains a loop of AMDAR soundings at RDU during the event from 18 UTC on 03/01 through
15 UTC on 03/02. During the 21 hour period from 18 UTC on 03/01 through 15 UTC on 03/02 there
were a total of 14 AMDAR soundings available at KRDU.
A Java Loop
of AMDAR soundings from 1819 UTC on Sunday, March 1 through 1406 UTC on Monday, March 2, 2009
that can be stopped, controlled and zoomed is available.

Forecasters used AMDAR data to supplement other observational data
to monitor the erosion of the mid level warm nose and to determine
the depth of the surface based shallow layer of cold air. Use of AMDAR data
was noted in the
1145 AM AFD on March 1 and the
345 PM AFD on March 1.

Snow Accumulation and Soil Temperatures

Light to moderate snow fell across much of central
North Carolina for several hours during the storm with a
varied snow accumulation pattern.
A subjective review of the event, notes that the snow accumulation was most efficient in locations with a cooler
soil temperature along with slightly cooler but subfreezing surface temperatures
and greater precipitation rates. The snow accumulated most readily
in the Triad area where accumulations ranged between 4 and 7 inches with 4 cm soil temperatures
in the upper 30s and surface temperatures falling to around 30 degrees during the time
in which snow was falling from the late evening of 3/1 through
the early morning hours of 3/2. Further east, across the Triangle area, the snow did accumulate
although there was considerable melting at the base of the snow accumulation near
the ground. Soil temperatures in the Triangle area
ranged between 39 and 42 degrees during the time
in which snow was falling from the late evening of 3/1 through
the early morning hours of 3/2.

Snow accumulated on most area roadways during the event with the
degree of accumulation varying based on the air temperature, ground temperature,
precipitation amount, precipitation rate, and the degree of direct sunshine available
to the roadway during days preceding the event (is the road in the shade for example).
One of the biggest limiting factors for snow accumulation was the
warm ground temperatures resulting from the warm and somewhat sunny days preceding the
event (highs at Raleigh-Durham were 65, 72, and 58 on February 26th through 28th).
The snow that accumulated on the roads in the Triangle area was very wet and melted from below
during the morning as shown in these photos from around 900 AM in North Raleigh (example 1 |
example 2 |
example 3). Most Triangle area roadways were
just wet by Monday afternoon with little or no slush or snow.

Hourly 4 inch (0.1m) Soil Temperatures

The image below (click on it to enlarge) shows the hourly 4 inch soil
temperatures at 6 locations across central North Carolina from midnight on
Saturday, February 28 through midnight on Wednesday, March 4. Note that for most locations the
soil temperatures were in the upper 40s to lower 50s just 36 hours prior to
the snow starting.

CoCoRaHS is a grassroots volunteer network of weather observers
of all ages and backgrounds working together to measure and map precipitation
(rain, hail and snow) in their local communities. By using low-cost measurement
tools, stressing training and education, and utilizing an interactive web-site,
CoCoRaHS aims to provide the highest quality data for natural resource, education
and research applications. The only requirements to join are an enthusiasm for
watching and reporting weather conditions and a desire to learn more about how
weather can effect and impact our lives. North Carolina joined the CoCoRaHS network in 2007.

The CoCoRaHS Web page provides the ability for CoCoRaHS observers to see their observations
mapped out in "real time", as well as providing a wealth of information for
our data users. The snow accumulation maps
from the CoCoRaHS web site (shown below) were a great resource for WFO RAH.

The CoCoRaHS intense snow report below was received by the NWS Raleigh just after
100 AM EST. The snow report from Forsyth County indicated that heavy snow had
fallen in that area with 3 inches of snowfall in the past 2 hours bringing
the storm total to near 4 inches. The report also provided some information
on the temperature and the local accumulation trends. The intense snow
report is especially important since it is very difficult to get accurate snow
accumulation reports late at night. The value of this kind of report cannot be overstated.

NZUS45 KBOU 020617
CCRAHS
Intense snow report from CoCoRAHS spotter:
03/02/2009 01:00 AM local time
County: Forsyth NC
Lewisville 4.2 N (number NC-FR-4)
Latitude: 36.154362
Longitude: -80.405506
3.00 inches of snowfall in the past 2 hrs
4.00 inches of snow on the ground
Comments: Snow was heavy most of the time from about 01Z through about
05Z but since then has steadily dropped off and is now very
light. Accumulation depends heavily on how exposed the
location is - 4" on fully exposed lawn away from trees and
the house, 3" on wooden deck near the house. Temperature has
just dropped below freezing and the wind has risen again
(from NNW) after being nearly calm most of the evening so
occasionally there's a little snow blowing and drifting off
the roof. Most of the trees are bent over and looking very
stressed, and there are quite a few small branches down
(about an inch in diameter).
Received NWS Boulder Sun Mar 1 23:17:51 2009 MST
Sent to WFOs: RAH,GSP,RNK
All of today's CoCoRAHS observations are in WRKCCR (Boulder and CRH only)
Or at http://www.cocorahs.org (click on reports)

Central North Carolina Snow Accumulation Totals

Triangle Area Snow Accumulation Totals

Archived Text Data from the Winter Storm

Select the desired product along with the date and click "Get Archive Data."
Date and time should be selected based on issuance time in GMT (Greenwich Mean Time which equals EST time + 5 hours).

Winter Storm Watches verified with an average lead time of nearly 40 hours and Winter Storm Warning
lead times were around 18 hours. There were no missed events and the Winter Weather Advisory area was well
defined and verified very well.

Forecasters used NCDOT and other highway traffic cameras to supplement the
limited traditional observations during the storm. Collecting information on snow
accumulation and snow impact during the overnight hours is often very difficult.
Traffic camera pictures
(example 1 |
example 2) were very helpful
in gauging the degree of now accumulation and its potential impact.

This event was another case in which forecasters utilized AMDAR aircraft
soundings to provide thermal profiles at GSO and RDU in between RAOB times.
This data set is an important resource during critical winter weather forecasts.
Forecasters during the event utilized the http://rucsoundings.noaa.gov/ website
and AWIPS to view these soundings during the event.

Briefings
were provided to local emergency managers and decision makers beginning on Friday, February 27,
via the new NWS
Raleigh Briefing Web Page and via other online conferencing software. Since this was an
event that occurred during and just after the weekend, the ability to share information with users
who may be at home or away from the office was invaluable.

Several CoCoRaHS intense snow reports were received by the NWS Raleigh.
These reports are especially important since it is very difficult to get accurate snow accumulation
reports late at night. The value of these reports cannot be overstated.

The National Operational
Hydrologic Remote Sensing Center provides comprehensive snow observations, analyses, data sets
and map products. These products can be used in the analysis of snow fall, snow cover, snow
water equivalent and in the anticipation of snow melt.

Warm weather preceeding the event along with warm ground temperatures
resulted in some melting of the snow as it accumulated across the Eastern Piedmont and Sandhills.

Isolated bands of snow that moved across the Northern Piedmont of North Carolina
during the morning of March 2, 2009 appeared to be a result of thermal instability in the boundary
layer that produced horizontal convective rolls.

Many of the images and graphics used in this review were provided by parties outside
of WFO RAH.
The upper air analysis images and Skew-T diagrams were obtained from the University of Wyoming.
Satellite data was obtained from National Environmental Satellite, Data, and Information Service.
Surface observations provided by the University of Wyoming.
The surface analysis graphic was obtained from the Hydrometeorological
Prediction Center.
Partial thickness analysis charts are courtesy of Dr. Michael Brennan and the N.C. State Meteorological Analysis and Prediction Laboratory.
AMDAR aircraft sounding data was obtained from the Earth System Research Laboratory - Global Systems Division.
Radar imagery was obtained from the National Weather Service web site. CoCoRaHS maps were provided by the CoCoRaHS organization.
Analog medium range guidance and background provided by the CIPS Winter Storm Analog Guidance page.
Ground temperatures and adjacent air temperature data provided by CRONOS from the N.C. State Climate Office. Photos are courtesy of Jonathan Blaes.